1. Field of the Invention
The present invention relates to an orbit correction apparatus for a charged particle beam optical system, and a charged particle beam apparatus, such as an electron microscope, having the same.
2. Description of the Related Art
A charged particle beam apparatus plays an important role in a wide field of nanotechnology. The charged particle beam apparatuses include a scanning electron microscope (SEM) that scans a focused beam of electrons over the surface of a sample, detects signal electrons from the surface of the sample, and makes the detected signal visible on an image display device; a transmission electron microscope that allows electrons to pass through a sample and focuses the diverging electrons through an electron lens into an image; an electron beam exposure system that subjects a sample to patterning by irradiating the surface of the sample with a shaped beam of electrons; and a focused ion beam apparatus that subjects a sample to a process by irradiating the sample with a focused ion beam. To bring these charged particle beams into convergence, the electron lens formed of rotationally symmetrical electrodes or magnetic poles is used because of having good controllability and processability.
A problem with such an electron lens system is electro-optical aberration. For example, an electromagnetic lens of rotationally symmetrical configuration functions as a convex lens because of having increased magnetic field intensity and hence an augmented converging action axially outwardly in the vicinity of a magnetic pole. Moreover, aberration that is a higher-order perturbation component of the lens causes a phenomenon in which a charged particle beam emitted from a given point does not converge to a point due to being dispersed depending on conditions of entrance of the beam into the lens. Even with the use of an ideal point source of light, the occurrence of the phenomenon leads to the occurrence of a finite spread of an image point, namely, what is known as beam defocusing, depending on a radiation angle distribution or a central orbit axis of the point source. The aberration can possibly cause deterioration in resolving power for sample observation using a focused beam of charged particles, or serious deterioration in accuracy of microfabrication.
According to perturbation aberration theory, it is known that, because of the occurrence of spherical aberration proportional to the third power of an incident angle α of a beam and chromatic aberration proportional to a deviation dV relative to accelerating energy V, the amount δ of departure of a beam orbit on the axis can be expressed in equation form as:δ=Csα3+CcdV/V+ . . .   (1)where Cs denotes a spherical aberration coefficient and Cc denotes a chromatic aberration coefficient. Other contributions take place outside the axis. At this point, beam defocusing occurs in accordance with the above equation in the event of α-dependent beam current distribution or energy dispersion. Generally, the charged particle beam apparatus requires a large current in order to increase its signal quantity or processing speed, and thus has to capture a wide range of charged particle beams emitted from a light source. That results in a wide distribution of orbit within a convergent lens, for which a trade-off is an increase in the amount of aberration, thus leading to a definition of performance in principle.
There have been proposals for methods for correcting the aberration. The methods include an aberration correction method that involves controlling the divergence and convergence of a multistage arrangement of regularly partitioned multipoles (see H. Rose, Nucl. Instrum. Meth. A 519, 12), and a multi-beam method that involves disposing a microlens array, splitting a charged particle beam into multiple beams, and performing orbit correction on the beams (see Japanese Patent Application Laid-Open Publication No. 2006-80155). Also included is a method that involves disposing a limiting aperture in ring zone form on the axis for the purposes of inhibiting aberration to some extent under a large current and also lessening the space-charge effect resulting from Coulomb repulsion or scattering particularly within a beam (see Japanese Patent Application Laid-Open Publication No. 2000-12454). An increase in the intensity of the charged particle beam raises also contributions made to space charge by electrons present on the axis, specifically a maximum intensity axis having a high current density. Thus, the concept of the above method is to use a doughnut-shaped aperture of axisymmetrical configuration and thereby increase the intensity of an electron source and hence an effective area for beam capture, rather than to capture the charged particle beam circularly about the axis. Discussions have been also made as to a method using an annular lens having an electrode disposed on the axis (see U.S. Pat. No. 3,100,260), as intended for a special electrooptic system.